1. Field of the Invention
This invention relates to IC fabrication and, in particular, IC fabrication utilizing strict design rules.
2. Art Background
It is presently contemplated that X-ray lithography will be employed for making integrated devices having design rules of 0.25 .mu.m and stricter while possible use at even 0.5 .mu.m has not been ruled out. (The design rule defines the lateral dimension of the smallest critical feature in the device pattern.) In such processes a substrate is coated with a material that is photosensitive to X-ray radiation. This material is exposed in a pattern defined by a mask for incident X-rays. The exposed photosensitive material is then developed by selective removal of either the exposed or unexposed portions. The resulting patterned substrate is further used through processes such as etching or deposition to advance towards completion of the device.
An X-ray mask for lithography generally includes a desired pattern of material that substantially attenuates X-ray radiation (absorbs more than 80% of incident radiation of wavelength suitable for inducing reaction in the photosensitive material) and a body for mechanical support of this attenuating material. However, a body of substantial thickness underlying the patterned region unacceptably attenuates X-ray transmission in regions that should be transmissive. Generally, to avoid this difficulty, the attenuating material is formed on a thin membrane (0.5 to 5.mu. thick) that is supported at its periphery by a thicker, typically ring shaped, region. Ideally, this membrane should be relatively robust, e.g., capable of withstanding pressures of 200 Torr for a 3 cm in diameter membrane of 2 .mu.m thickness, and economic to manufacture. However, membranes in general, and X-ray masks in particular, typically do not satisfy these requirements.
The most widely investigated X-ray mask involves the formation of a heavily boron doped silicon membrane. This mask is fabricated by forming a thin region of heavily boron doped silicon on a silicon wafer. The periphery of the silicon substrate on the major surface opposite from the doped silicon is covered with an etch resistant material. The subtrate is then immersed in an etchant that selectively removes undoped silicon relative to boron doped material. (Selectively etching in this context is an etch rate of the substrate at least 50, preferably 500, more preferably 1000 times faster than the membrane.) With appropriate selectivity, the undoped silicon of the substrate exposed to the etchant is removed while the thin layer of boron doped silicon remains. However, sufficient selectivity to avoid substantial thinning of the boron doped silicon is not obtained unless the boron dopant concentration is at least 5.times.10.sup.19 cm.sup.-3 and the boron doped silicon material is epitaxially grown as a single crystal on the silicon substrate.
The formation of a relatively heavily doped epitaxial silicon region is not an inexpensive fabrication procedure. A heavily boron doped silicon membrane is relatively transparent to X-rays and has a tensile stress in the range 3.times.10.sup.8 to 5.times.10.sup.9 dynes/cm.sup.2 ensuring freedom from membrane deformation. However, the single crystalline silicon membrane is susceptible to fracture along crystallographic planes. Thus, these membranes are neither robust nor are they economic to manufacture. As a result, presently contemplated X-ray lithography masks are not totally satisfactory.